Merge pull request #359 from liuchong/master

Add my docker images:

README.md

@@ -221,6 +221,7 @@ See also [Friends of Rust](https://www.rust-lang.org/friends.html) (organization
 * Docker
   * [emk/rust-musl-builder](https://github.com/emk/rust-musl-builder) — Docker images for compiling static Rust binaries using musl-libc and musl-gcc, with static versions of useful C libraries
   * [kpcyrd/mini-docker-rust](https://github.com/kpcyrd/mini-docker-rust) — An example project for very small rust docker images [<img src="https://api.travis-ci.org/kpcyrd/mini-docker-rust.svg?branch=master">](https://travis-ci.org/kpcyrd/mini-docker-rust)
+  * [liuchong/docker-rustup](https://github.com/liuchong/docker-rustup) — a multiple version (with musl tools) Rust Docker image
   * [messense/rust-musl-cross](https://github.com/messense/rust-musl-cross) - Docker images for compiling static Rust binaries using musl-cross [<img src="https://api.travis-ci.org/messense/rust-musl-cross.svg?branch=master">](https://travis-ci.org/messense/rust-musl-cross)
   * [rust-lang-nursery/docker-rust](https://github.com/rust-lang-nursery/docker-rust) — the official Rust Docker image
 * Google App Engine